Response to inoculation of groundnuts grown under irrigation in soil containing indigenous rhizobia

Five strains of Bradyrhizobium spp. highly effective on groundnuts (Arachis hypogaea L.) were used to inoculate groundnuts grown under irrigation in each of two soils. Both soils contained small numbers of indigenous rhizobia capable of nodulating groundnuts. Root nodules typed serologically by means of the ELISA technique at ca. 40 and 70 days after planting, indicated fair to excellent nodulation by at least four of the inoculant strains; nodules on uninoculated plants tended to be fewer, especially when mineral nitrogen was applied at planting. The percentage nodules formed by an inoculant strain on either tap or secondary roots did not differ appreciably at the two sampling times. At harvest, various yield parameters failed to show a significant effect of inoculation or nitrogen application on the groundnut yield. The results indicate the problem of obtaining a response to inoculation when even low numbers of rhizobia effective on groundnuts are present in the soil.